Windows 7 all the way. If your current system is running Vista pretty well, then it will run Windows 7 as good as, if not better than Vista.
So much has been improved, including the task bar and various other features within the UI. But, performance isn't worse than Vista. For me, it was actually better than Vista, including boot up and log in time.
Also, UAC has really been toned down as far as notifications. Plus, you can tailor it to your preferences, but at least the default mode is less intrusive than it was in Vista.
I say, bite the bullet and go with Windows 7. It is so much better.